1. Field of the Invention
This invention relates to an imaging lens system for use as a photo-taking lens or the like, and particularly is suitably used as the photo-taking lens of a camera such as a digital still camera.
2. Description of Related Art
In recent years, digital still cameras have been spreading as the image input apparatuses of computers. In such digital cameras, it is usually practiced to AD-convert an output signal from a solid state image pickup element (photoelectric conversion element) such as a CCD sensor or a CMOS sensor into image data, subject it to a compressing process such as JPEG, and record it in a recording medium such as a flash memory. The compressed data recorded in this manner is evolved on a computer, and thereafter is displayed on a monitor or the like.
In such digital still cameras, the higher definition of a photographed image and the downsizing of the apparatus have become tasks and above all, the compatibility of high resolving power and compactness is required of a photographing optical system. Particularly to provide a thin type camera with importance attached to the portability thereof, a photographing optical system having a short full length is required. To shorten the full length of the optical system, it is advantageous to use a photo-taking lens having the smallest possible number of components.
As photo-taking lenses of long back focal length suitable for digital still cameras, there are known numerous photo-taking lenses of the so-called retrofocus type comprised of a front lens unit of negative power and a rear lens unit of positive power. Particularly as a photo-taking lens of the retrofocus type comprised of a small number of components, there is one of which the front lens unit is comprised of a single negative lens.
As such a photo-taking lens of the retrofocus type of which the front lens unit is comprised of a single negative lens, there is disclosed in U.S. Pat. No. 5,418,649. A photo-taking lens of three-unit three-lens construction comprising, in succession from the object side, a negative lens, a positive lens and a positive lens. In another embodiment in U.S. Pat. No. 5,418,649, there is disclosed a photo-taking lens of three-unit four-lens construction comprising, in succession from the object side, a negative lens, a positive lens and a positive cemented lens.
Further, in Japanese Patent Publication No. 7-122692 (corresponding U.S. Pat. No. 4,984,878), there is disclosed a photo-taking lens of four-unit four lens construction comprising, in succession from the object side, a negative lens, a positive lens, a positive lens and a positive lens. In Japanese Patent Publication No. 5-37288, there is disclosed a photo-taking lens of four-unit four-lens construction comprising, in succession from the object side, a negative lens, a positive lens, a negative lens and a positive lens. In Japanese Patent Publication No. 5-20724, there is disclosed a photo-taking lens of four-unit four-lens construction comprising, in succession from the object side, a negative lens, a positive lens, a positive lens and a negative lens. In Japanese Patent Publication No. 61-46807 (corresponding U.S. Pat. No. 4,493,537), there is disclosed a photo-taking lens of four-unit five-lens construction comprising, in succession from the object side, a negative lens, a positive lens, a positive lens and a positive cemented lens. In Japanese Patent Application Laid-Open No. 9-189856 (corresponding U.S. Pat. No. 5,706,141), there is disclosed a photo-taking lens of four-unit five-lens construction comprising, in succession from the object side, a negative lens, a positive lens, a negative cemented lens and a positive lens. In U.S. Pat. No. 4,146,304, there is disclosed a photo-taking lens of four-unit five-lens construction comprising, in succession from the object side, a negative lens, a positive lens, a positive cemented lens and a positive lens. In U.S. Pat. No. 4,674,844, there is disclosed a photo-taking lens of four-unit five-lens construction comprising, in succession from the object side, a negative lens, a positive lens, a positive cemented lens and a negative lens. In Japanese Patent Publication No. 60-32165 (corresponding U.S. Pat. No. 4,130,349), there is disclosed a photo-taking lens of five-unit five-lens construction comprising, in succession from the object side, a negative lens, a positive lens, a negative lens, a positive lens and a positive lens.
All of the photo-taking lens disclosed in the above-mentioned publications are of the retrofocus type, and in them, the first lens is a negative front lens unit and the second and subsequent lenses constitute a positive rear lens unit.
The above-described conventional photo-taking lenses, however, have been insufficient in both of performance and size as ones for use in the recent digital still cameras of which higher resolution and a smaller size are required.